1. Field of the Invention
This invention relates to apparatus for measuring pressures, for example, differential pressures developed by flowing fluids. More particularly, this invention relates to apparatus and techniques for calibrating the span of pressure measuring instruments.
2. Description of the Prior Art
Pressure measuring instruments often are installed in locations where they are subject to widely varying environmental conditions, and thus it is not uncommon for the instrument calibration to shift unpredictably from time to time, thereby causing erroneous measurements. Accordingly, there has developed a need for some means of readily calibrating such instruments periodically, or when appropriate, so as to assure accurate readings. U.S. Pat. Nos. 4,604,891 and 4,708,010 assigned to the assignee of this application disclose apparatus for assisting in so calibrating the span of a pressure measuring instrument.
The apparatus shown in those patents comprises a reference pressure device located at the instrument and operable upon activation to develop a pressure pulse of predetermined magnitude. In one preferred embodiment, the device includes a vertically-oriented cylindrical tube containing a fill-liquid. A solid metal ball is raised by a solenoid to the top of the tube and then released. The ball drops under the force of gravity through the fill-liquid to produce a pressure pulse of essentially constant magnitude over much of the drop distance. This pressure pulse is applied to the sensing means of the instrument, and the resulting output signal is monitored to determine whether there has been a shift in the span calibration. If there has, circuitry associated with the instrument is adjusted to compensate for such shift, thereby to assure an accurate pressure reading.
Although the apparatus disclosed in the above two patents is fully capable of producing the desired reference pressure pulse, it has been found that certain additional operational characteristics would be especially useful in satisfying important user needs in industrial process instrumentation systems. For example, it is highly desirable to furnish users with apparatus which is "intrinsically safe", as determined by specified certifying agents empowered to rule on these matters. With the original designs as described in the above patents, this would be difficult to achieve because the relatively high power required to lift the ball (and an attached weight) necessarily introduced a large amount of electrical energy into the nearly-sealed spaces of the instrument interior. Also, the overall system of the original design was somewhat complex, requiring multiple circuits between the instrument and other stations connected thereto.